This plant disease is caused by a number of closely related fungi (Puccinia species). The rust-colored pustules are composed of millions of microscopic spores. Some of the spores spend the winter on iris leaves that have not died back entirely, while others overwinter on other kinds of plants. Infection usually starts in the spring as soon as conditions are favorable for plant growth. The spores are spread to healthy plants by splashing water and wind. Because iris varieties vary greatly in their susceptibility to rust, some may be killed prematurely, while others may not be affected. Rust is greatly favored by wet weather.
Spray infected plants with a fungicide containing mancozeb or triadimefon. Repeat the spray 2 or 3 more times at intervals of 7 to 10 days. Remove and destroy old and dying iris leaves in the fall. Water in the morning to allow the foliage a chance to dry out before nightfall. Plant rust-resistant varieties, if available.
Plant Care for Iris